Update README.md

Author: manhtai

README.md

@@ -46,7 +46,7 @@ We believe in a transparent and fair world, so we open-source the app and
 you get a trial version for your personal use, but if you find the app helpful,
 please make a purchase.
 
-Purchase page: https://plainlab.gumroad.com/l/plainmerge
+Purchase page: https://gum.co/plainmerge
 
 ---